Pressurized water reactors (PWRs) use fuel assemblies which contain a nuclear fuel, such as enriched uranium dioxide, to produce a nuclear chain reaction. The nuclear chain reaction is moderated in several ways, including maintaining specific fuel enrichment levels, maintaining a specific geometry of the fissionable nuclear fuel assemblies, and placing absorber material through the nuclear reactor core.
The purpose of control rod absorbers is to slow or capture neutrons as the neutrons traverse the nuclear reactor. The absorber material allows sections of the core which potentially have greater amounts of nuclear activity to be reduced to more moderate levels. Because the control rod absorbers perform this task, the overall performance of the nuclear reactor is more consistent and the reactor is able to run with a greater degree of operational safety.
Over time, conventional control absorber rods start to degrade. This degradation leads to the sections of the core potentially becoming more radiologically active than other sections of the core. The degradation then leads to localized “hot spots” in the reactor core and may require nuclear power plant operators to limit overall reactor operation to maintain safe operating margins, thereby negatively impacting the economic operation of the facility. When the rods become severely degraded, the reactor cannot be safely operated, necessitating rod replacement. As these absorber assemblies which contain the rods are located in the reactor core itself, the absorber assembly replacement occurs during a reactor outage. The replacement of these absorber rods and assemblies must be performed very carefully as the materials themselves are highly radioactive after their residence in the reactor core. The removal of the assemblies from the core is performed remotely by use of a crane and the removal is therefore a difficult and expensive operation. Ultimate disposal of the highly radioactive components removed is also very expensive as such highly radioactive waste must be cooled with cooling water for a long period of time. The components may then be stored in a “dry” condition in a specially prepared cask with a gaseous cooled interior. All of these disposal costs increases the overall cost of operation of the facility.
Control rod assemblies may be provided in several various arrangements. “Black” clusters may be used in a reactor, wherein such “black” clusters are highly absorbent to neutrons traversing the nuclear reactor core. Other clusters, commonly known as “gray” clusters, may also be positioned around the reactor. The “gray” clusters are less absorbent than the “black” clusters and are therefore used in core positions that do not need as much radiation attenuation as other sections of the core. “Black” clusters are generally constituted of rods containing materials which are highly absorbent to neutrons, such as silver-indium-cadmium alloys.
Absorber rods and assemblies degrade through a variety of degradation mechanisms. Absorber rods made of silver-indium-cadmium alloys are subject to both creep and swelling under irradiation. Boron carbide alloy based absorber rods undergo a large amount of swelling under irradiation and as a result are not heavily used in portions of rods inserted into high activity core areas, such as the bottom of absorber rods. For this reason, silver-indium-cadmium alloys are primarily used in the bottom parts of control rods. For silver-indium-cadmium alloys, the chief degradation mechanism limiting operability is diametral expansion. Diametral expansion of rodlets is a result of irradiation-induced expansion, thermal creep and thermal expansion. The overall geometric shape of the absorber assembly causes degradation to occur most frequently at the lower tip of a rod because of a high fluence exposure and high stresses due to the force exerted from an internal absorber stack of the rod.
This is a need to produce a rod with absorber material which will limit diametral expansion.
This is also a need to produce an absorber rod which will have an increased service life in the nuclear core compared to conventional absorber rods and that will not have to be replaced as often as other absorber rod units.
There is also a need to produce an absorber rod assembly which will limit the amount of nuclear waste generated for a nuclear reactor.